Yarn tension measuring devices of the basic type described above are known in various embodiments. With many operations in the textile industry, for example in the rewinding of a spinning cop on a large volume cheese, there is a requirement for maintaining the yarn tension approximately constant.
In automatic cheese winders, various suggestions have therefore previously been made and devices have been produced in the past for continuously regulating the yarn tension, for compensating for changes in yarn tension and for maintaining predetermined values of the yarn tension. Various control principles are known for this purpose which, depending on the requirements, can be applied independently of each other or in combination with each other.
One principle involves the regulation of the rotational speed (rpm) of the take-up bobbin in order to maintain the yarn tension essentially constant by adjusting the yarn tension during the work process.
Another principle for the regulation of yarn tension utilizes a disk brake system, for example, to provide the yarn with the desired tension while traveling onto the take-up bobbin by braking the yarn to a greater or lesser degree when passing through the brake disks in order to provide the desired tension.
The function of both systems consists in automatically regulating the values determining the yarn tension as a function of changes in the yarn tension by automatically compensating for every temporarily occurring difference in the yarn tension from a selected normal yarn tension. Thus, the sensing and, therefore, the measurement or control of the instantaneously prevailing yarn tension is a prerequisite for all regulating steps of these systems. In this connection, the total functioning and, particularly, the precision of the regulating system for maintaining the yarn tension constant is ultimately dependent on how accurately and with what amount of inertia the continuous control or measurement of the yarn tension can be performed.
A three point system is mainly employed for measuring the yarn tension of a traveling yarn, wherein the force of the yarn on a measuring eye which deflects the yarn is determined. To be able to make a determination of the tension from this force, the angle of contact of the yarn at the measuring eye must be known. So that this angle of contact is exactly maintained, the yarn rests on two further yarn guide elements, in addition to the measuring eye, which are also part of the yarn guide device. In this connection, it is known that with dynamically operating sensors the measuring eye should be as lightweight as possible in order to obtain a high threshold frequency of the sensor.
For example, a yarn tension measuring device is known from German Patent Publication DE 35 06 698 A1, which has a compact housing having stationary yarn guides in the form of small rollers disposed on the yarn inlet and outlet of the housing. A diabolo, which has a low coefficient of friction, is attached on a resilient sheet metal support piece, clamped on one end, at a defined angle to, and a defined distance from, these yarn guides. In this manner, the resilient sheet metal piece can be deflected by the effect of the yarn tension. A permanent magnet is fastened underneath the diabolo, also on a movable sheet metal piece, which follows every movement caused by the change in the yarn tension. A Hall sensor is stationarily fastened on the housing at a defined distance from the permanent magnet at the level of the magnet. Each movement of the magnet is registered by the Hall sensor, which is connected to a control device by means of an electric conductor so that the desired yarn tension is entered in the control device. With excessively high or excessively low tension, the control device transmits a signal which results, for example, in an adjustment of the yarn tension at the respective winding head.
German Patent Publication DE 40 25 005 A1 describes a device in which a semiconductor thin film pressure sensor is employed for measuring the yarn tension. In this case, this integrated circuit (IC) component is arranged inside a protective housing and is biased under pressure by means of a mechanical lever element which is in contact with the yarn. On its end, the lever element has a measuring eye in which the yarn slides. In addition, stationary yarn guides are disposed at a distance in advance of and following the movable measuring eye in the customary manner described.
Both the sensor device in accordance with German Patent Publication DE 40 25 005 A1 and the sensor device in accordance with German Patent Publication DE 35 06 698 A1 have the disadvantage that the yarn guide device of the yarn tension sensor is very sensitive to changes in the yarn tension, in particular abrupt losses of the yarn tension. In case of such abrupt losses of the yarn tension there is the danger that the yarn slides out of the movable, relatively small measuring eye or diabolo, which is open in order to make the insertion of the yarn possible, which results in a complete loss of function of the device, which can only be remedied by the manual intervention of an operator.